The present invention is related to the field of data communications, and more particularly to the transmission of local area network data traffic through a synchronous communications network.
Traditional data communications among computers has been carried out using local-area networks (LANs). Ethernet LANs, for example, have been very widely used in the data communications field. Generally, data communication over LANs has employed data units or “packets” having a variable size. Explicit address information is included in a header portion of each packet to identify the recipient. Specialized control logic at the various nodes of a network are responsible for detecting incoming packets, temporarily storing them in variable-size buffers, and determining from the address information which node or nodes the packet is destined for.
As LAN technology has developed, there has also been development in the technologies used in traditional telephony communications. Pulse code modulation (PCM) of voice signals and synchronous time-division multiplexed (TDM) transmission channels have been in use for many years. With advances in optical fiber technology, communications carriers have devised very high data rate signals for fiber optic transmission that carry thousands of individual TDM channels using hierarchical TDM techniques. In particular, a set of standardized synchronous transport signals (STSs) are utilized in networks that adhere to Synchronous Optical Network (SONET) standards. Synchronous digital signals having data rates ranging from about 51 Mb/s to over 100 Gb/s are defined in SONET, each signal generally incorporating an integer number of basic “STS-1” signals.
There has been increasing interest in and need for communications equipment that can interface with traditional and emerging LANs, on the one hand, and the high-speed synchronous communications networks of the type traditionally deployed in telephony communications. The telephony networks, for example, are used for inter-LAN communications in wide-area networks (WANs), and therefore special interfaces are required to translate between LAN hardware and protocols and the hardware and protocols of the synchronous networks. Additionally, SONET-compliant equipment has been incorporated into portions of private and semi-private networks where the expense of such equipment is justified by the performance it provides, for example in backbone segments that are required to carry very high volumes of data traffic.
In these hybrid networks, there has been increased use of techniques that can be classified as “LAN emulation”. In a typical application, two or more disjoint LAN segments, for example segments residing in different buildings, are connected by one or more high-speed network segments of the type traditionally used in longer-haul networks such as the telephony networks. For example, one or more fiber optic links carrying SONET traffic may be used for such a high-speed link. The equipment that interfaces the separate LAN segments to the high-speed segments operates such that the collection of separate LAN segments appear to the connected host computers as a single LAN. This type of operation has numerous benefits, including the protection of investments in LAN hardware and software while providing greater connectivity and network capacity than would be possible using LAN technology alone.
In some networks of the type described above, the high-speed segments may provide data transport services to a number of different sets of users. For example, different businesses within a building or complex may utilize the high-speed network. There may be several independent emulated LANs, for example, that share the use of the high-speed network. In such cases, it may be necessary to allocate the usable capacity or bandwidth of the high-speed network among these multiple entities, to ensure that each enjoys a specified capacity without regard to the use of the network by the other entities.
Another issue to be addressed in networks of the foregoing type is the manner in which data traffic should be carried. The synchronous signals in the high-speed network are typically organized into fixed-size frames. The signal may retain a TDM aspect, in which case there must be a conversion between the connected LANs and discrete channels of the signal. In some cases, some part or all of the synchronous signal may have a “concatenated” format, meaning that the boundaries that typically exist among discrete TDM channels are not present, and the signal can be treated as a single fixed-rate stream available to carry data traffic. An example of such a signal is an Optical Carrier 3c, or OC-3c, which is a 155 Mb/s signal having no TDM sub-structure.
Protocols, such as High-Level Data Link Control (HDLC), have been used to transmit variable-length frames in an otherwise undifferentiated signal such as an OC-3c signal. Like other framing protocols, HDLC employs special “escape” characters and “escape sequences” that convey signaling information such as frame boundaries. One problem with such techniques is that the pattern of the escape character generally occurs in the data stream being framed, and if transmitted without modification would be erroneously interpreted as an escape character. To deal with this situation, a technique called “character stuffing” is used. Basically, every occurrence of the escape character in the data stream is replaced with a multi-character escape sequence that signifies that the receiver should insert the pattern of the escape character in the received data stream.
One significant drawback of employing HDLC or similar framing is the unpredictable expansion of the data rate that results from character stuffing. If a user data signal is specified to have a given fixed rate, for example, then the rate in the network is increased in proportion to the rate at which the escape character pattern appears in the data. Although on average such expansion may be very small, there may be realistic worst-case patterns that can result in expansion of 10% or more for non-negligible periods of time. If bandwidth is allocated based on such worst-case traffic, the link is generally under-utilized, which is inefficient. If bandwidth allocation is based on average traffic, then there may be an unacceptable rate of traffic loss when worst-case data patterns occur.
A number of links have been used to carry a single stream of packets or LAN frames. The stream is divided into separate logical channels, each of which is carried over a corresponding link. This technique has the effect of providing a desired overall transmission capacity by employing a number of lower-capacity links. Systems employing such techniques are generally referred to as “multilink” systems. It is generally necessary to append control information to the data transmitted over each link to enable a receiver to reconstruct the original data stream. On each link, it is necessary to frame or otherwise delineate the data and control information just as in the case of non-multilink transmission. When HDLC or similar framing is employed, the above-described problem of excessive rate expansion may exist.